In rotating machinery, any imbalance of the involved rotating-shaft system induces significant vibrations near its primary critical speed. To suppress such vibrations, the rotating-shaft system may be balanced to cancel the imbalance inducing the vibrations.
No matter to what degree the imbalance is corrected, if any sympathetic vibration can be still produced, some procedure to suppress the vibration should be applied to the rotating shaft system. Up to the present, the prior-art of vibration control methods for a rotating-shaft system has been proposed as listed below.
The simplest method provides a suppressing effect by elastically suspending a bearing using an elastic body. For example, the elastic body employed in this method may a rubber vibration isolator as described in Tallian, T. E., and Gustafsson, O. G., ASLE Trans., Vol. 8, 3 (1965), or a blade spring as described in Masayoshi Itoh, Kikai Sekkei [Machine Design] 6,12 (1962), 30. Kirk, R. G., and Gunter, E. J., J. Engineering for Industry, 2(1972), 221, and Ota, H., and Kanabe, Y., J. Applied Mechanics, Vol. 98, 1 (1976), 144, teach that a suppressing effect can be produced by applying the dynamic vibration absorber theory to an elastic suspended system.
Iwata, Nova, Kiron [Articles on Mechanical Engineering], 49-446, 10 (1983), 1897 realizes the specified values of the optimized parameters defined by the dynamic vibration absorber theory using an active control.
In the art of aircraft gas turbine engines, a squeezing film damper bearing is widely used to inhibit vibrations, as described in, e.g., Morton, P. G., Proc. Instn. Mech. Eng., 180 (1965), 295, and Hirsch, F., and Childs, D., Mech. Eng. 5 (1984), 66.
Recently, an active vibration control of a rotating-shaft system, which is supported by a magnetic bearing using a variety of control theories, has been established, as described in, e.g., Japanese Patent Early-Publication No. 2003-166535.
In particular, methods of vibration suppression of a centrifugal separator have been developed and realized, as listed below.                1) As shown in Japanese Patent Early-Publication No. 07-088401, a separation cylinder is supported by a rotating shaft to be rotated like a pendulum. The rotating shaft is provided with a buffer device in which a hydraulic damper is combined with a rubber damper.        2) As shown in Japanese Patent Early-Publication No. 10-180147, a cylinder whose center axis is a rotation axis houses a plurality of balls. A ball balancer automatic corrects any imbalance by moving the balls.        3) Japanese Patent Early-Publication No. 2003-144977 discloses a means for stopping the rotation of a motor when a driving shaft jointed to a rotor contacts a vibration detector.        
Further, methods of vibration suppression of a washing machine are being developed and realized, as listed below.                1) Japanese Patent Early-Publication No. 05-131075 discloses a method of vibration suppression of a washing machine. In this method, the damping force of a suspension, which supports a water tank of the washing machine, is varied by means of varying the damping force based on the output of a vibration sensor, which detects the vibrations of the washing machine.        2) Japanese Patent Early-Publication No. 11-207082 discloses a method of vibration suppression of a washing machine. In this method, the washing machine is suspended from a vibration isolator, which is provided with a hanging bar, an upper sliding member, and a sliding cylinder. On the inner periphery of the sliding cylinder, a viscoelastic material having thixotropic properties is used.        
Unfortunately, the forgoing devices and methods of vibration suppression involve the disadvantages listed below.                1) Because the elastic constant of the rubber damper can be significantly varied as a result of variations in the age or temperature, controlling and maintaining the associated parameters with high precision involve difficulties.        2) To provide an elastically suspended structure with a bearing, the arrangements of the rotating machinery must be modified, and this results in complex arrangements. It may compromise the safety of the rotating machinery.        3) Even if the optimized values are determined based on the dynamic vibration absorber theory, controlling the parameters, typically, the damping coefficient, at the optimized values, involves difficulties.        4) The forgoing methods using the squeeze film damper bearing or the variety of control theories require complex and large devices and thus increase the cost if they are applied to common rotating machinery.        5) Because most of the forgoing methods are designed to suppress the steady-state vibration and thus to provide damping effects thereon, their advantages for unsteady-state vibrations are unknown.        
Accordingly, there are needs to provide a device and a method that can readily suppress the vibrations of the rotating-shaft system by a simple arrangement without fine adjustments for the parameters.